1. Field of the Invention
The present invention is related generally to a transmitter apparatus, a receiver apparatus and a base station implemented with the transmitter and receiver apparatuses which are communicated with each other by making use of an orthogonal frequency division demultiplexing modulation and a spectrum spreading modulation in combination.
2. Prior Art
In recent years, it becomes an important issue to cope with interference due to delay spread through multipaths while the signal transmission rates of wireless communication systems have been increased. OFDM (Orthogonal Frequency Division Multiplexing) is a technique with which the problem of the interference due to delay spread can be solved. OFDM is a block-oriented modulation scheme that maps data symbols onto a plurality of orthogonal sub-carriers separated by a distance to provide excellent resistance characteristics against the interference due to delay spread through the multipaths. Also, OFDM is capable of compacting a substantially larger number of sub-carriers, as compared with a conventional FDM (Frequency Division Multiplexing), to provide an extremely high utilization of the frequency resources. OFDM is employed as a signal transmission scheme for digital broadcast in Europe, U.S.A. and Japan, and, in addition to this, determined to be employed as part of the standard for wireless system such as HIPER-LAN/2 (Europe), IEEE802.11a (U.S.A.), MMAC (Japan) which are the next generations of the mobile communication systems.
Meanwhile, the next generations of the wireless communication system are expected to support high speed signal transmission rates in the order of several Mbps to several tens of Mbps with a wider bandwidth exclusively occupied by each channel. Because of this, it becomes furthermore important to improve the utilization of the frequency resources as compared with conventional systems. Furthermore, because of the limitation on the frequency resources as available, a technical object resides in how to determine an effective arrangement of the frequency resources and an effective arrangement of the respective cells in the cellular system.
Furthermore, the next generations of the wireless communication system are expected to support different signal transmission rates to accommodate a variety of information types for use in multimedia communications which require different levels of QoS (Quality of Service). The support for different signal transmission rates can be implemented by the use of different modulation systems and different encoding rates. Such a system capable of supporting different signal transmission rates is called as “a multi-rate supporting system” in the following description. The following table shows the relationship among the transfer speeds, the encoding rates, the modulation schema and the receiver sensitivities.
RELATIONSHIP AMONG TRANSFER SPEED, ENCODING RATE,MODULATION SCHEMA AND RECEIVER SENSITIVITIESTRANSFERENCODINGMODULATIONRECEIVERMODESPEEDRATESCHEMESENSITIVITYM1 6 MbpsBPSK1/2−82 dBmM2 9 MbpsBPSK3/4−81 dBmM312 MbpsQPSK1/2−79 dBmM418 MbpsQPSK3/4−77 dBmM527 Mbps16 QAM 9/16−74 dBmM636 Mbps16 QAM3/4−70 dBmM754 Mbps64 QAM3/4−65 dBm
In the case of this example, there are seven modes M1 to M7 are implemented. Needless to say, a good wireless transmission condition is required for communication at a high speed transmission rate. As described in the above table, it is necessary to secure a higher reception electric field strength in order to change the transition mode from the low rate transition mode to the high rate transition mode, i.e., from the mode M1 to the mode M7. Inversely, when the transition mode is changed from the high rate transition mode to the low rate transition mode, i.e., from the mode M7 to the mode M1, the reception electric field strength as required is lowered. Namely, in the case of the multi-rate supporting system, it is possible to vary the extent (coverage) of the area (cell) to which radio waves can be effectively transmitted from one base station by changing the signal transmission rate. More specifically speaking, it is possible to expand the coverage of a cell by decreasing the signal transmission rate. Such a system capable of changing the coverage of a cell is called as “a dynamic cell structure system” in the following description.
Exemplary prior art dynamic cell structure systems are described in “Studies of Zone Generation Algorithm in Adaptive Variable Zone Structure System”, Institute of Electronics, Information and Communication Engineers, B-5-204, 1998 and described in “Studies of Adaptive Variable Zone Structure System Implemented with a Directional Antenna in a Base Station”, Communications Society Conference, B-5-81, 1998. In the case of these exemplary prior art techniques, it is accomplished to lessen the load on a base station due to disparity of the number of mobile stations to be linked with the base station and decrease the distance between adjacent zones utilizing the same frequency by making use of an adaptive array antenna and adaptively modifying the profile of the zone in accordance with the distribution of mobile stations.
Furthermore, another exemplary prior art dynamic cell structure system is described in “Area Configuration Method in Multi-Rate Compatible High Speed Wireless LAN”, Communications Society Conference, B-5-89, 1999. The coverage of a cell is changed by modifying the zone profile in the case of the exemplary prior art technique as described above by making use of an adaptive array antenna. Contrary to this, in accordance with the above described B-5-89 reference, the coverage of a cell is changed by varying the transmission rate of the beacon signal.
The system becomes more flexible as the variable range of the coverage of a cell is increased in the dynamic cell structure system. For this reason, it is a technical issue how to expand the variable range of the coverage of a cell.
Furthermore, it is inevitable that the interference with an adjacent cell is increased while the coverage of a cell is expanded. In other words, the expansion of the coverage is closely related to the arrangement of cells in the cellular system. From this fact, there is a problem as to how to arrange cells in the dynamic zone structure.
Also, the utilization of the frequency resources has to be improved in a wireless communication system. Particularly, in the case of the wireless communication system in the next generations where the occupied signal bandwidth per channel is designed to be broad, the frequency resources as available are limited so that an appropriate system design is required to accomplish highly utilization of the frequency resources.
The intelligent antenna (smart antenna) is a wireless communication system improving the utilization of channels. The intelligent antenna technology has been explained, for example, in “Intelligent Antenna Technology”, Communications Society Conference Vol.1, TB-5-1, 1999. The exemplary prior art dynamic cell structure systems as described in “Studies of Zone Generation Algorithm in Adaptive Variable Zone Structure System” and “Studies of Adaptive Variable Zone Structure System Implemented with a Directional Antenna in a Base station” are also examples of application of the intelligent antenna.
The exemplary prior art as described in “Unnecessary Waves Suppression Characteristics for Multi Carrier—CMA Adaptive Array” is an example of application of the intelligent antenna to the OFDM system. In this example, the respective signals as received through a plurality of antenna elements are appropriately weighted and then synthesized by means of a synthesizer. The signals as synthesized are converted into the signals in the frequency domain by means of FFT. The weight factor is determined on the basis of CMA (Constant Modules Algorithm) in order to make equal all the amplitudes of the respective sub-carriers. In the case where the reception electric power of the target waves is sufficiently large, the control scheme on the basis of CMA is considered to be effective.
The reception electric field strength as required is depending on the signal transmission rate in the case of the multi-rate supporting system. For this reason, it is important how to effectively control the weight factors given to an adaptive array antenna in such an environment where different coverages are given to users.
Also, in the case of such a system in which the communication range is expanded by the antenna gain as obtained by directing the beam from an adaptive array antenna to the target mobile station, it is impossible to continue communication unless the antenna is directed to the mobile station. In other words, while communication can be continued without controlling the direction of the antenna in the case where the mobile station is located within the service area, it is impossible to establish communication with a mobile station located outside of the service area unless the antenna is directed to the mobile station by beam control. The antenna is controlled in order to appropriately direct the beam on the basis of the information obtained from the received signals. Accordingly, it is impossible to obtain information necessary for taking appropriate control of the antenna in order to direct a beam to the mobile station that is located in a remote position where a communication link can be established only by securing a necessary antenna gain through the adaptive array antenna directed to that mobile station.
In this manner, the next generations of the wireless communication system are expected to support high speed signal transmission rates in the order of several Mbps to several tens of Mbps with a wider bandwidth exclusively occupied by each channel. Because of this, it becomes indispensable to improve the utilization of the frequency resources as compared with conventional systems. Furthermore, because of the limitation on the frequency resources as available, the technical object resides in the arrangement of the frequency resources and the arrangement of the respective cells in the cellular system.
Furthermore, the system becomes more flexible as the variable range of the coverage of a cell is increased in the dynamic cell structure system. For this reason, it is a technical issue to expand the variable range of the coverage of a cell. Furthermore, it is inevitable that the interference with an adjacent cell is increased while the coverage of a cell is expanded. Namely, it shall not be the case that the interference with an adjacent cell is increased while the coverage of a cell is expanded. From this fact, it is a technical issue how to arrange cells in the dynamic zone structure.
Also, it is important how to effectively control the weight factors given to an adaptive array antenna in which the utilization of the frequency resources is improved by making use of the adaptive array antenna. Furthermore, it is impossible to obtain information necessary for taking appropriate control of the antenna in order to direct a beam to the mobile station that is located in a remote position where a communication link can be established only by securing a necessary antenna gain through the adaptive array antenna directed to that mobile station. In consequence, it is an important technical problem how to determine the initial position of a remote mobile station (the initial position determination).